A bar of the type being described is used as an insulation component in the structure of the oil-barrier insulation of electric induction devices or inductances. The structure of the oil-barrier insulation includes either insulation cylinders or insulation sheets made of an insulating sheet material, called the barriers, which are spaced from one another by spacer bars of the type described. The reliability and quality of the oil-barrier insulation is heavily dependent on the regularity of the spaces between the barriers, and the regularity of these spaces is dependent on the accuracy of the dimensions of the spacer bars. The structures of the oil-barrier insulation generally fall within three types, namely, with planar arrangement of the barriers, with concentric arrangement of the barriers, and with barriers of arbitrary shapes. In most cases the oil-barrier insulation of an electric induction device is interposed between the low-voltage winding and the high-voltage winding, and also arranged about the high-voltage winding. As long as the low-voltage winding tends to contract under electrodynamic loads, while the high-voltage winding tends to expand, more often than not the oil-barrier insulation is not subjected to substantial mechanical loads. This lack of effect of high electrodynamic loads upon the structure of the oil-barrier insulation is explained by the fact that the windings of an electric induction device themselves are expected to be protected against the effect of the electrodynamic loads. This requirement according to which the windings should alter neither their shape nor their geometrical (overall) dimensions under the load is incorporated in the design of an electric induction device. Neither is the oil-barrier insulation arranged outside the high-voltage winding, on the oil tank wall and in other places subjected to excessive mechanical loads.
The spacer insulation bars of the oil-barrier insulation can be eventually loaded by way of their plane-parallel compression between the insulation cylinders brought about by the own weight of the oil-barrier insulation which is not considerable, or else by efforts applied to them at the assembly stage; these efforts, however, are not too difficult to either eliminate altogether or at least to alleviate.
An electric induction device, e.g. a power transformer or a reactor, has a number of current-conducting leads or taps usually in the form of either copper buses or cables. The leads or taps are fastened on the active part of an induction device with the aid of insulation bars. These bars, in addition to affording the required insulation, are also expected to have sufficient mechanical strength. The conventional practice is to make them of strips of hardwood, e.g. beech, or else of hard wood-plastic laminates, i.e. of materials offering adequate insulation and strength properties. Beech wood, however, is both a costly and hard-to-get material whose reproduction in nature is a lengthy process, whereas suitable plastic laminates are likewise costly.
An insulation bar for the oil-barrier insulation is generally made as a multilayer strip having its layers cemented to one another.
The manufacturing process of such bars is highly labor -consuming on account of its multioperational character. The operations involved are pattern-cutting a sheet of an insulating material, forming a multilayer strip, applying the adhesive onto the layers, drying in a compressed state, removing burrs and overflowing adhesive, stripping, etc. This manufacturing process is energy-consuming, too, requiring as it does for its implementation elevated temperatures and considerable pressures. A drawback of this technology is also the tendency of the bars being manufactured to get warped and delaminated during the drying and heat/vacuum treatment steps, which results in a considerable number of rejects.
A bar of the kind being described involves a heavy input of the insulating material, as its cross-section is solid, i.e. completely filled with the material.
More refined structurally and less material-consuming is an insulation bar made as a lengthwise rolled up strip having its longitudinal edges bent towards each other.
A disadvantage of this structure, however, is its open cross-sectional outline, which, on the one hand, impairs its mechanical strength, and on the other hand may result in a varying thickness of the bar itself on account of eventual warping and shrinking of the insulating material. A varying thickness of an insulating bar is intolerable when the bar is used as a spacer between the barriers of the oil-barrier insulation, as it impairs the dielectric strength of the insulation, and, hence, adversely affects the performance and service reliabiality of the electric induction device, as a whole.
There is known an insulation bar for supporting electric conductors (the leads or taps) in an electric induction device (GB, A, No. 2109639). This support is a box-section beam made of insulating board. However, this bar also has an open cross-sectional outline, impairing its mechanical strength. The use of additional reinforcing devices for enhancing the mechanical strength of the bar is a labor-consuming operation.